Why can't a BoxLayout be shared whereas a FlowLayout can?

Question

I am confused by the following.

Case A

1. Set the layout manager for a JFrame as a BoxLayout.
2. Add a JButton to the JFrame container.
3. Compile.
4. Run.
5. Exception thrown: "Exception in thread "AWT-EventQueue-0" java.awt.AWTError: BoxLayout can't be shared"

Case B

1. Set the layout manager for a JFrame as a FlowLayout.
2. Add a JButton to the JFrame container.
3. Compile
4. Run
5. No exception thrown.

Why does Case A throw the exception and Case B does not? Why do FlowLayout and BoxLayout behave differently in this regard? What does it mean in Case A by the "BoxLayout can't be shared"?

I have read a few similar threads on this website about this exception message but I'm still confused about the comparison to FlowLayout and what the exception message means exactly.

Answer 1

Simple answer

Basically, "BoxLayout can't be shared" in this situation means that you're trying to make your JFrame and its contentPane() share the same BoxLayout object.

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Advanced Explanation

When setting the Layout for a JFrame, it implicitely calls getContentPane().setLayout(manager) so you are actually setting the layout for the contentPane() and not for the frame itself.

This brings us to how the containers are checked within BoxLayout and FlowLayout.

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FlowLayout

This layout does not have a constructor that will take the container as a parameter and thus will not, at the making of the object, consider the container. Neither has this class a container instance variable.

BoxLayout

BoxLayout on the contrary has a constructor that takes the container as a parameter and stocks it within the instance variable target. This is done to check it later within the layoutContainer(container) method. It has a checkContainer(container) method that will verify if the instance variable is equals to the container given in parameter. It throws throw new AWTError("BoxLayout can't be shared"); if it is not the case.

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This was an introduction to the following explaination.

As said within the first paragraph, JFrame.setLayout(LayoutManager) will call JFrame.getContentPane().setLayout(LayoutManager) and thus the Layout is set on the contentPane() which is a JPanel by default.

Check out the constructor of BoxLayout(container, int) and ask yourself :

Now I know that the Layout is set on the JPanel (contentPane()) and not the JFrame itself, which parameter will I give to this constructor?

Is it a good idea to give it the JFrame itself? Knowing that it is not the Component which on the Layout has been set?

The answer is : of course it is not a good idea.

Here is the correct way to achieve this :

JFrame frame = new JFrame();
frame.setLayout(new BoxLayout(frame.getContentPane(), ...);

Why?

Because we know now that the container is the contentPane() and that eventually, when adding a component for example, a check is going to happen and the parameter within the constructor must be the exact same object as the component where on the layout is set which does not happens with FlowLayout for example.

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Sources

BoxLayout

• BoxLayout(Container, int)
• BoxLayout.checkContainer(Container)
• BoxLayout.layoutContainer(Container)

FlowLayout

• FlowLayout.layoutContainer(Container)